Yes, slight dirt on the surface will come up tan or brown. A deep mustard yellow color, which may be faint, is a result of a chemical reaction with the surface of phenolic resin. A phenolic bead from almost 100 years ago would give this color unless the beads had been heat treated, which is possible. But all the cherry amber jewelry beads Iíve tested gave a positive result in this test.
If you can do the bead reamer test you can confirm the ammonia test result. Ream the hole rapidly and take a sniff of the sawdust. Also observe the color of the sawdust by wiping it on a white cloth. If the bead is amber, the odor will be piney. If musty, medicinal, you have phenolic. If stinky plastic smell, itís another plastic, probably polyester.
If youíd like to mail me one Iíll check it for you and send it back.